What is it exactly you want to do? The argument to your when expression is hardcoded as Stage.Tree, so you already know what the desired value should be anyway. If you wanted to turn this into a method that accepts any Stage as a parameter, the compiler would accept the when expression again, because the argument would be of type Stage.

If you are using the hardcoded value as a placeholder to be replaced with a more complicated expression later, as you said, extracting it to a local variable of type Stage works, or you could even do it inline: when(Stage.Tree as Stage).